| Popis: |
The Barren Measures Formation of the Pranhita-Godavari Basin is distinctly different from its type area, the Jharia coalfield, in terms of (i) the absence of organic-rich shale, ferruginous shale and ironstones, (ii) predominance of coarse-grained sandstones in comparison to the finer grained fractions, (iii) the abundant signatures of early diagenetic changes, and cement within the sandstones and mudstones, and (iv) the occurrence of tidal influence among coarse-grained sandstones. In-depth facies analysis from selected outcrop sections record eight distinct facies types, distributed in three facies associations, namely, (i) the conglomerate-sandstone facies association (BM-CS), (ii) the sandstone-mudstone facies association (BM-SM) and (iii) the mudstone facies association (BM-M). The vertically stacked, multi-storeyed nature of the cross stratified sandstones of the BM-CS facies association in the lower part of the successions with paucity of fine-grained sediments of the BM-M facies association points to a braided river deposit. A mixed fluvial-tidal zone is interpreted from the occurrence of tide-influenced facies association (BM-SM), which overlies the fluvial deposits. Overall fining upwards succession depicting fluvial braided river deposit in the lower part, changing to a mixed fluvial-tidal transitional deposit in the upper part points, to a zone of tidal limit near the headward part of a tide-dominated estuary. Overall, the successions in the study area show a change in stratal stacking pattern with high channel to overbank ratio in the lower part of the succession, resulting in development of High Amalgamation Systems Tracts (HAST), to a stratal stacking pattern with low channel to overbank ratio in the upper part of the succession, resulting in development of Low Amalgamation Systems Tracts (LAST). Mostly, the HAST deposits are devoid of tidal signatures, whereas, the LAST deposits bear signatures of tidal influence, dominantly towards the top part of the successions.Varied stratal stacking patterns manifest modifications of the river equilibrium profile (base level),with changing accommodation, as a result of interplay of the climate, source-area uplift, and different autocyclic controls. Abundance of the tide influenced facies association (BM-SM) towards the top part of the successions, suggests landward encroachment of the tidal limit, causing frequent flooding and rising of the base level, leading to a changing stratal stacking pattern.This possibly signify subtle change in the river morphology, which took place in response to the interplay of base level fluctuations, sediment supply, and energy flux within the Middle Permian Barren Measures Formation of the Pranhita-Godavari Basin. |
